The goal of this project is to substantiate a role for the red blood cell (RBC) in the regulation of microvascular perfusion in the peripheral microcirculation. The appropriate matching of oxygen supply to demand requires a sensor of tissue oxygen need, and effector of alterations in oxygen supply to meet those needs. The P.I. s earlier studies have indicated that RBC oxygen content is more important than partial pressure of oxygen, pO2, for the maintenance of capillary oxygen supply in severe hypoxia. Since the only portion of the oxygen transport pathway directly influenced by oxygen content is the hemoglobin in the RBC, it was suggested that the RBC must be involved in sensing oxygen demand and altering blood flow appropriately. The level of oxygen content of the RBC's in blood vessels at a particular point in the tissue is directly linked to the level of oxygen utilization by the tissue. If the mobile RBC were able to sense oxygen need and bring about decreases in vascular caliber, this would provide an efficient means of increasing blood flow and oxygen delivery wherever and whenever the need might arise. This would eliminate the need for a diverse network of sensing sites throughout the vasculature. Since ATP is present in RBCs, which is released in response to low pO2 and low pH, and when applied intraluminally into arterioles and venules induces a conducted vasodilator response, it was hypothesized that the RBC is a sensor of tissue oxygen requirements, and an initiator of a conducted vasodilator response via its release of ATP, which enables the appropriate matching of oxygen supply with demand. Three basic approaches will be used to evaluate this hypothesis: 1) Using isolated RBC-perfused hamster retractor muscle arterioles, the relationship between low extraluminal pO2, ATP release and vessel diameter will be evaluated; 2) The mechanisms by which the ATP-induced vasodilation is conducted upstream along the microvasculature will be investigated; and 3) The role of the cystic fibrosis transmembrane conductance regulator (CFTR) and associated signal transduction pathways in the release of ATP from the RBC will be determined. These findings will be combined into a physiological model for the regulation of blood flow distribution to meet tissue needs, a model which would fill the role of the long sought after metabolic regulator of blood flow distribution.